1. Field of the Invention
This invention relates generally to an apparatus and method for mounting printing plates. More specifically, this invention relates to an apparatus and method for quickly and accurately mounting flexible printing plates used in flexographic and letterpress printing processes onto printing plate cylinders.
2. Background Information
In one form of the printing process, printing is effected by photopolymer or rubber printing plates mounted on printing cylinders. The paper or label to be printed is impressed on the inked printing plate as the cylinder rotates. The cylinder on which the printing plates are mounted is generally called the plate or printing cylinder. The printing plate is typically attached to the printing cylinder with dual-sided tape. The quality of the printing job depends, in a large measure, on the care in which pre-press preparations are carried out. Pre-press preparations include mounting the flexible plate on the printing cylinder, checking color registration, and proofing. Proofing indicates the appearance of the final reproduction, affording a means to check the mounting of the plates for color sequence, spacing requirements, layout, and gear size, as well as copy and color separation. Mounting of the plate is accomplished with the use of commercially available mounting or mounting-proofing machines designed for this purpose.
The mounting of photopolymer or other printing plates onto printing cylinders for subsequent printing requires a high degree of accuracy in the alignment of the plate onto the cylinder. The image on the printing plate must be perfectly square and in register with the printing cylinder in order to print square and in register on the paper or label. When multiple colors are printed or there is the requirement of superimposition of images, the various colors or images are added sequentially via multiple runs through the printing press. Accordingly, it is very important that in each case the printing plate which is adding the successive color or image be synchronized with the preceding plate or plates so that the colors or images are accurately superimposed. If this does not occur, the resulting paper or label may be blurry or fuzzy in certain areas. The arrangement of the printing plates in the exact predetermined relationship with one another requires that their angular as well as their transverse position on the printing plate support apparatus be accurately determined prior to mounting. After the plates are mounted onto the cylinders and short test runs of the printing process are performed, the printing press operator must stop the press to carefully examine the test results. If the results are acceptable, then the operator commences with the printing operation. However, if the results are unacceptable after careful inspection, the operator must remove the plates from the cylinders and start over from the beginning. One can see that this activity is very time-consuming, especially if the printing press operator's skill in aligning the printing plate on the printing cylinder is anything but excellent every time. Substantial down-time in running the printing press results from inaccurate plate mounting, thereby decreasing the efficiency of the overall printing process.
One common method to effect the alignment of the printing plates with respect to the printing cylinder involves the drawing of a line around the printing cylinder. This line is then aligned by eye with a longitudinal line along the length of the photopolymer or other printing plate. The plate is then wrapped around the cylinder. This method is somewhat accurate but can be extremely time consuming for the operator. This leads to delay between print runs.
An early solution to the plate mounting problem is described in U.S. Pat. No. 3,390,633, issued to Boughton. In the Boughton device, the printing plate was laid on a flat surface on top of a rectangular flexible film. The flexible film had rods secured to it at each end. The printing cylinder was then placed on the printing plate, and the operator rolled the printing plate onto the printing cylinder by grasping the rods at either end of the flexible film and curling the ends around the cylinder. This method and apparatus was very error-prone and inaccurate.
Another device used for mounting photopolymer or other printing plates is disclosed in U.S. Pat. No. 3,616,055, issued to Mages. In the optical plate mounter of Mages, the printing cylinder is placed in a fixed relationship to the plate mounting device. The printing cylinder is attached at its ends in front of and slightly below a table top. The printing plate is laid upside down on the clear glass table top. The table top has a grid etched into the glass to assist in alignment of the printing plate. A projector is disposed above the table to shine light down onto the table to assist the operator in aligning the printing plate. However, one disadvantage to the Mages device is that when the operator leans forward to align the plate, his or her head usually casts a shadow over the very area where he or she is attempting to work. This shadow interferes with the operator's vision when aligning the printing plate, thereby negatively affecting the plate alignment process. Once the printing plate is aligned, the operator manually turns the printing cylinder while feeding the printing plate off the table and onto the rotating printing cylinder. One skilled in the art can readily see that this method is very error-prone and results in misalignment of the printing plate on many occasions.
A similar device is disclosed in U.S. Pat. No. 4,449,452, issued to Mansell. The Mansell device also requires the operator to manually rotate the printing cylinder to receive a printing plate held to a flat surface by vacuum suction. Again, the printing cylinder is held in place in front of and below the table top surface. The printing plate is brought to the printing cylinder as the cylinder is turned by hand. Bringing the plate to the cylinder in this manner results in skewed or inaccurate plate mounting an unacceptable percentage of the time. The reason this method is so error-prone is that it is very difficult to retain proper alignment of the printing plate to the printing cylinder as both elements are being moved by the operator.
Another printing plate mounter is shown in U.S. Pat. No. 4,743,324, issued to Boyce et al. This plate mounter uses a table with two separable sections on which to lay the printing plate. The printing cylinder is held below the table top positioned so that as one section of the table top is moved laterally, the printing plate can be slid off the edge of the other table top section and wrapped around the printing cylinder as the cylinder is rotated. This device and its operation are still problematic for the plate mounting process because they still do not solve the deficiencies of the prior art discussed immediately above in that the printing plate is still being brought to the printing cylinder by hand. Hence, misalignment and skewing still result.
Other devices appear in the prior art which utilize more complicated apparatus such as microprocessors and the like, or rely on registration pins and holes to accurately align the printing plates. These devices are too complicated, too costly, or are simply not applicable to the mainstream paper and label printing trade. In addition, many printing shops today have multiple presses designed for specific sizes of printing plates and associated printing cylinders. A plate mounter capable of mounting printing plates on one size of printing cylinder may not work for other sizes of printing cylinders. At times, a printing shop may have to purchase several different plate mounters to ensure it has the capability to mount various sized printing cylinders.
Clearly, what is needed in the flexographic and letterpress printing industry is a simple, relatively inexpensive plate mounter that will allow an operator to quickly and accurately mount a flexible printing plate on a print cylinder. Furthermore, this plate mounter should accommodate a variety of sizes of printing plates and printing cylinders. The plate mounter should allow for both horizontal and vertical alignment, and should be easy to use. The present invention provides a significant advance over the prior art in meeting the needs of the flexographic and letterpress printing industry.